The present invention relates generally to machine tools, and particularly to a system to be incorporated with a machine tool for detecting excessive torque load, or both torque load and thrust load, to which the cutting tool may be subjected during the operation of the machine tool, in order to protect the cutting tool from destruction due to such overload.
A variety of overload detection systems have been suggested and used with machine tools. Among such known systems is the one described and claimed in Japanese Unexamined Patent Publication No. 59-142049. This prior art device teaches the use of two gears for detecting abnormal torque load on the cutting tool. The first of the two gears is mounted fast on the toolholder for joint rotation therewith. The second gear is coupled to the first gear via elastic means that is subject to torsional deformation in proportion with the cutting torque. The phases of the two gears are detected by respective phase detectors such as proximity switches. The cutting torque is computed electronically on the basis of the phase difference between the gears for comparison with a normal range of torque values.
An objection to this prior art device is that it needs two phase detectors, one for each gear. The device would become far simpler and less expensive in construction if it could do with a single phase detector. Moreover, as used in the prior art device, the two phase detectors have proved to be very easy to give rise to errors in the computation of the cutting torque, as the cuttings produced during machine tool operation unavoidably attach to and accumulate on them.
Japanese Utility Model Publication No. 54-10060 proposes another cutting torque detection system. This second known system comprises a rotary member mounted fast on the spindle, a screw stud on a chuck support, and a torsion spring between the rotary member and the chuck support. The screw stud is rotatably engaged with the rotary member. The torsion spring yields when the cutting torque exceeds a predetermined limit, resulting in a relative phase displacement between rotary member and chuck support and, consequently, in the axial displacement of the screw stud. This axial displacement of the screw stud indicates the excessive torque on the cutting tool.
This second conventional system is also objectionable for the following reasons. The rotary member and the chuck support must be capable of relative phase displacement through a considerably great angle in order to cause axial displacement of the screw stud upon development of excessive cutting torque. Such excessive cutting torque must therefore by very high in comparison with the yielding point of the torsion spring. An additional inconvenience is that it takes an inconveniently prolonged time for the rotary member and the chuck support to undergo relative phase displacement through the required great angle. Consequently, this prior art system fails to set the drive spindle out of rotation instantly upon development of any excessive torque.
Japanese Patent Publication No. 63-28742 teaches how to detect excessive thrust load on the cutting tool. To this end a spool attached to a collet chuck is mounted within a hollow spindle for axial displacement relative to the same. The spindle has formed therein three fluid passageways to make up a thrust sensing valve in combination with the spool. The fluid passageways communicate with pressure operated switches which are selectively actuated upon linear displacement of the spool relative to the spindle in the event of the development of an excessive thrust load on the cutting tool.
This third prior art system is subject to the drawback that complex construction is required solely for the detection of abnormal thrust load. Additional means are needed for the detection of excessive torque load, making the machine tool even more complex and expensive in construction.